ARROW-2250: [Python] Do not create a subprocess for plasma but just use existing process
ARROW-2250: [Python] Do not create a subprocess for plasma but just use existing process

Author: Mitar <mitar.git@tnode.com>

Closes #1705 from mitar/ARROW-2250 and squashes the following commits:

4e71d445 <Mitar> ARROW-2250:  Do not create a subprocess for plasma but just use existing process.

yufeldman/arrow,itaiin/arrow,icexelloss/arrow,apache/arrow,itaiin/arrow,icexelloss/arrow,xhochy/arrow,majetideepak/arrow,xhochy/arrow,majetideepak/arrow,icexelloss/arrow,pcmoritz/arrow,apache/arrow,renesugar/arrow,xhochy/arrow,wesm/arrow,kou/arrow,wagavulin/arrow,xhochy/arrow,pcmoritz/arrow,wesm/arrow,kou/arrow,dremio/arrow,wagavulin/arrow,dremio/arrow,icexelloss/arrow,xhochy/arrow,laurentgo/arrow,renesugar/arrow,icexelloss/arrow,tebeka/arrow,yufeldman/arrow,wesm/arrow,majetideepak/arrow,cpcloud/arrow,majetideepak/arrow,wagavulin/arrow,wesm/arrow,cpcloud/arrow,majetideepak/arrow,xhochy/arrow,tebeka/arrow,cpcloud/arrow,apache/arrow,renesugar/arrow,icexelloss/arrow,laurentgo/arrow,yufeldman/arrow,laurentgo/arrow,itaiin/arrow,cpcloud/arrow,xhochy/arrow,renesugar/arrow,cpcloud/arrow,xhochy/arrow,kou/arrow,itaiin/arrow,yufeldman/arrow,cpcloud/arrow,renesugar/arrow,renesugar/arrow,laurentgo/arrow,laurentgo/arrow,wagavulin/arrow,yufeldman/arrow,wesm/arrow,laurentgo/arrow,itaiin/arrow,dremio/arrow,wesm/arrow,laurentgo/arrow,wagavulin/arrow,majetideepak/arrow,kou/arrow,yufeldman/arrow,apache/arrow,itaiin/arrow,cpcloud/arrow,laurentgo/arrow,cpcloud/arrow,xhochy/arrow,wagavulin/arrow,pcmoritz/arrow,pcmoritz/arrow,itaiin/arrow,wesm/arrow,laurentgo/arrow,cpcloud/arrow,apache/arrow,itaiin/arrow,renesugar/arrow,yufeldman/arrow,itaiin/arrow,icexelloss/arrow,majetideepak/arrow,renesugar/arrow,xhochy/arrow,kou/arrow,itaiin/arrow,laurentgo/arrow,apache/arrow,kou/arrow,laurentgo/arrow,wagavulin/arrow,renesugar/arrow,apache/arrow,xhochy/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,pcmoritz/arrow,wesm/arrow,apache/arrow,majetideepak/arrow,pcmoritz/arrow,cpcloud/arrow,dremio/arrow,wagavulin/arrow,laurentgo/arrow,xhochy/arrow,wesm/arrow,tebeka/arrow,apache/arrow,pcmoritz/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,pcmoritz/arrow,tebeka/arrow,wagavulin/arrow,itaiin/arrow,pcmoritz/arrow,yufeldman/arrow,wesm/arrow,wesm/arrow,dremio/arrow,laurentgo/arrow,wagavulin/arrow,renesugar/arrow,cpcloud/arrow,kou/arrow,majetideepak/arrow,wesm/arrow,dremio/arrow,itaiin/arrow,kou/arrow,itaiin/arrow,itaiin/arrow,renesugar/arrow,renesugar/arrow,dremio/arrow,wagavulin/arrow,apache/arrow,tebeka/arrow,apache/arrow,xhochy/arrow,wesm/arrow,tebeka/arrow,kou/arrow,apache/arrow,dremio/arrow,renesugar/arrow,kou/arrow,icexelloss/arrow,majetideepak/arrow,pcmoritz/arrow,cpcloud/arrow,xhochy/arrow,laurentgo/arrow,majetideepak/arrow,majetideepak/arrow,wagavulin/arrow,apache/arrow,xhochy/arrow,wesm/arrow,pcmoritz/arrow,yufeldman/arrow,tebeka/arrow,dremio/arrow,yufeldman/arrow,kou/arrow,cpcloud/arrow,tebeka/arrow,dremio/arrow,majetideepak/arrow,pcmoritz/arrow,pcmoritz/arrow,icexelloss/arrow,apache/arrow,dremio/arrow,majetideepak/arrow,renesugar/arrow,pcmoritz/arrow,pcmoritz/arrow,majetideepak/arrow,kou/arrow,icexelloss/arrow,tebeka/arrow,kou/arrow,itaiin/arrow,yufeldman/arrow,renesugar/arrow,tebeka/arrow